The present invention relates to an acceleration switch, and more particularly, to an acceleration switch that includes a movable portion, which has a movable electrode, and a fixed electrode and that moves the movable portion when acceleration is applied so that the movable electrode contacts the fixed electrode.
Many automobiles are nowadays equipped with air bag systems. An air bag system generally includes an air bag, an ignitor, and an electronic control unit (ECU). The ECU includes an acceleration sensor, which detects a sudden change in acceleration when the vehicle collides. A semiconductor acceleration sensor is used as such type of an acceleration sensor. The semiconductor acceleration sensor includes, for example, a strain gauge arranged on a beam, which supports a mass. The ECU activates the ignitor when determining that the applied acceleration is greater than or equal to a predetermined value. The thermal expansion of heated air instantaneously inflates a folded air bag.
The ECU may function erroneously when affected by electromagnetic waves generated by surrounding devices. Therefore, the employment of a mechanical acceleration switch (i.e., safing sensor) in addition to an electronic acceleration sensor has been proposed. The mechanical acceleration switch is less affected by the electromagnetic waves than the electronic acceleration sensor.
FIGS. 1A to 1C schematically show the structure of a prior art acceleration switch 51.
The acceleration switch 51 includes a silicon chip 52 and a substrate 53, which are attached to each other. The silicon chip 52 has a hollow portion 52a, in which a generally rectangular parallelepiped inertial weight 54 is arranged. A beam 55 is provided on each long side of the inertial weight 54 at a position offset from the middle of the long side. The beams 55 connect the inertial weight 54 and the silicon chip 52. The beams 55 support the inertial weight 54 at a position offset from the center (center of gravity) of the inertial weight 54. Two movable electrodes 56, 57 are arranged close to each other on the lower surface of the inertial weight 54 at a generally middle part of the distal end that is on the side farther from the beams 55.
A hollow portion 53a is defined in the upper surface of the substrate 53. A fixed electrode 58 is arranged in the hollow portion 53a at a position corresponding to the movable electrodes, 56, 57. The movable electrodes 56, 57 are normally spaced from the fixed electrode 58.
When acceleration is applied to the acceleration switch 51, inertial force is applied to the inertial weight 54 such that the inertial weight 54 pivots about the beam 55 in a downward direction (the direction indicated by arrow G in FIG. 1A). When the acceleration applied to the acceleration switch 51 becomes greater than or equal to a predetermined value, the inertial weight 54 pivots in a direction indicated by arrow F in FIG. 1A, and the movable electrodes 56, 57 contact the fixed electrode 58. When the acceleration is small, the inertial weight 54 does not pivot about the beam 55. Thus, the fixed electrode 58 does not contact the movable electrode 56. The acceleration switch 51 operates only when the applied acceleration is greater than or equal to the predetermined value.
When acceleration is applied to the acceleration switch 51 from a direction other than a predetermined detection direction, inertial force is applied to the inertial weight 54 from a direction indicated by arrow G in FIG. 2A. In such case, the inertial weight 54 pivots in a twisted state, as shown in FIG. 2A. In such state, an edge of the lower surface of the inertial weight 54 first contacts the substrate 53, as shown in FIG. 2B. This restricts the movement of the inertial weight 54. The two movable electrodes 56, 57, which are on the same plane, may not contact the fixed electrode 58. In other words, contact failure may occur in the acceleration switch 51 when acceleration is applied from a direction other than the predetermined detection direction.
It is an objective of the present invention to provide an acceleration switch, which operation is guaranteed even when acceleration is applied to the acceleration switch from direction other than the a predetermined detection direction.
One perspective of the present invention provides an acceleration switch including a movable portion, which has a movable electrode, and a fixed electrode, in which movement of the movable portion in accordance with the application of acceleration causes the movable electrode to contact the fixed electrode. The acceleration switch has the movable portion, which includes an inertial weight moved in accordance with the application of acceleration, a beam portion for pivotably supporting the inertial weight, and a plurality of flexible plates arranged in the inertial weight, each of which has distal end, in which the movable electrode is located, and is flexed separately from one another.
In this structure, since the plurality of flexible plates, each provided with the movable electrode, flex separately, the movable electrodes move separately when the flexible plate are flexed. Therefore, even if only one of the movable electrodes contacts the fixed electrode when acceleration is applied to the acceleration switch from an unexpected direction, the other movable electrode moves separately from the former movable electrode and contacts the fixed electrode. Therefore, the activation of the acceleration switch is guaranteed even when acceleration is applied from an unexpected direction.